An oceanographer will tell Congress on Thursday that the Obama administration was “misleading” when it claimed that about three- quarters of the oil that gushed from a broken BP PLC (BP, BP.LN) well in the Gulf of Mexico had been broken down or cleaned up.

Ian MacDonald, an oceanographer at Florida State University, will tell a U.S. House Energy and Commerce subcommittee that only 10% of oil discharged into the ocean was “actually removed from the ocean.” In a report released earlier this month and touted by the White House, the government emphasized different numbers, saying that 17% of the the oil released by the well had been collected without ever reaching the ocean and about half had dissolved or been dispersed.

The government’s report “gives the impression that the clean-up efforts were more effective than they actually were,” MacDonald will tell the subcommittee on energy and environment. He will say that the report “mixes very different categories together,” such as oil that can harm the environment in the future and oil that “posed no such threat” once it was pumped into tankers. The prepared testimony was reviewed by Dow Jones Newswires.

On Wednesday, National Oceanic and Atmospheric Administration chief Jane Lubchenco defended the government’s estimates, saying that “we stand by the calculations that we released recently.” She said that the government was “going forward” with “additional monitoring” and would change its estimates if “new information should come to the fore.”

Earlier this month, a team led by the U.S. Interior Department and NOAA said that of 4.9 million barrels released by the well, just over one fourth was a ” residual amount” that was either on or just below the surface as a light sheen and weathered tar balls or had washed ashore.

“We really cannot check whether this number should actually be 36% of 19%,” MacDonald will say. He will say that the report does not provide any citations or formulas that would allow “an independent reviewer to determine where these numbers actually come from.”

MacDonald will also challenge the government’s statement that the oil released into the ocean is biodegrading quickly.

“Science simply does not know how quickly or slowly oil will degrade either in surface waters of in the deep waters of the Gulf,” MacDonald will say. He will say that preliminary evidence suggests “a slow rate of degradation.” That contradicts the government’s statement earlier this month that “oil from the BP Deepwater Horizon spill is biodegrading quickly.”

MacDonald will also say that oil that has resisted dispersion and evaporation “will be very persistent” and “remain potentially harmful for decades.”

MacDonald will say that the gas released by the spill “should not be ignored.” He will say that fish exposed to concentrated methane “have exhibited mortality and neurological damage.”

He will also say that he is concerned about the ability of the Gulf of Mexico to withstand the shock of the oil spill.

“My greatest concern is that portions of the ecosystem may experience “tipping point” effects that overwhelm resiliency,” MacDonald will say. While “we can hope” that the spill’s distance from shore and its depth “will mitigate the impact,” scientists “have to watch with utmost scrutiny.”

MacDonald also will say that the Gulf of Mexico must be “first in line” for payments made by BP to compensate for damage from the spill. That could set off a conflict with residents of the Gulf region, who are also seeking compensation for the damage to their livelihoods.

“Much as I sympathize with the economic hardship caused by the BP discharge and desire that restitution be paid, a big part–the biggest part–of our response must put the Gulf herself first in line for repayment,” he will say.